The present invention relates generally to an antenna system for a satellite, and more particularly, to a dual-reflector antenna system having an elliptical rim shape.
Communication satellites use various types of antenna systems for communication. Phased array antennas are often used as well as antenna systems that use dual reflectors. Dual reflector antenna systems include a main reflector and a sub-reflector. A feed is used to radiate the communication signals to the sub-reflector which is then reflected to the main reflector. The main reflector then directs the communication signal to the desired communication target. The main reflector shapes the desired beam into a particular shape and direction in the far-field.
One problem with a dual reflector antenna system is that undesirable signals originating from the dual reflector antenna system may be present in the far field. Two types of undesirable signals present in the far field are signals that are radiated directly from the feed and signals that are scattered by the sub-reflector rim. Typically, the antenna geometry controls the amount that the feed contributes to the far field. However, signal scatter from the sub-reflector rim can coherently add in a particular direction to form a xe2x80x9cgain effect.xe2x80x9d The signal scatter from the sub-reflector is caused by the rim edge. Although the reflected signal from the rim of the sub-reflector is smaller in intensity, it can interfere with the primary signal resulting in multi-path effects which can lead to ripple over the operating frequency band as well as ripple in the desired beam. In many communication systems it is required that a null signal or side lobe region be generated. These signals are usually of low signal strength. This is done for example, to prevent signal coverage in a particular direction of the far-field. The far-fields scatter from the sub-reflector can be significantly higher than the primary null signal or side lobe area signals.
One way in which to reduce undesirable signals originating from the feed and sub-reflector rim is to modify the antenna geometry. This may be accomplished by repositioning the feed and sub-reflector so that the coherent detracted field from the sub-reflector rim is pointed away from the direction of the desired be null. One draw back to this approach is that because of mechanical constraints of the spacecraft, arranging the sub-reflector and feed may not always be feasible.
It would therefore be desirable to improve the geometry of a sub-reflector system to reduce the amount of undesirable signal diffracted by the sub-reflector rim.
It is therefore one object of the invention to change the sub-reflector shape to reduce the amount of radiation reflecting from the rim thereof.
It one aspect of the invention an antenna system comprises a feed generating a communication signal. A sub-reflector is positioned to reflect the communication""s signal to form a sub-reflective signal. A main reflector is positioned to reflect the sub-reflective signal. The reflector has an elliptical rim.
In a further aspect of the invention, the sub-reflector has a super-elliptical rim shape.
One advantage of the present invention is that the elliptical rim shape may be used for various reflector configurations such as a Cassegranian or Gregorian. Another advantage of the invention is that increased null depth and side lobe characteristics are obtained. In one construction configuration, a null depth was increased by a factor of sixteen.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.